1. Field of the Invention
The present invention is directed to an electrolytic capacitor, and more particularly to an electrolytic capacitor having a novel floating anode between the cathode and the powered anode, resulting in a single capacitor with a working voltage double that of the anode formation voltage.
2. Related Art
Capacitors are energy storage devices usually consisting of two conducting surfaces (plates) separated by an insulator (the dielectric). When voltage is applied to the capacitor plates, an electrical field is created between them. The strength of the electrical field is directly proportional to the voltage on the plates and inversely proportional to the distance between them. The ability of the plates to hold opposite and equal charges when a voltage is applied to them is termed capacitance.
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of Implantable Cardioverter Defibrillators (ICDs), also referred to as implantable defibrillators, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Electrolytic capacitors (either wound roll or flat) are commonly used in ICDs, because they have the most ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Aluminum electrolytic capacitors, having plates comprised of aluminum foil, are the most common type currently employed in implantable defibrillators. However foils of other conventional valve metals such as titanium, tantalum, magnesium, niobium, zirconium and zinc can also be used. Conventional electrolytic capacitors are constructed with a powered anode foil plate at a positive potential with respect to a negatively charged cathode foil plate. The anode foil is typically roughened or etched to enhance surface area and a barrier oxide layer is formed continuously over the anode foil surface to support the intended voltage. This oxide layer is the dielectric of the electrolytic capacitor. The powered anode and cathode are separated by a kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. The separator material impregnated with the liquid electrolyte in conjunction with the negatively charged cathode foil functions as the negative terminal of the capacitor. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably where the salt is the salt of the same weak acid, employed in a polyhydroxy alcohol solvent. Typically, the electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent.
In the case of a cylindrical or wound roll electrolytic capacitor, the surface area of the foils are made as large as possible by employing very thin rolls of foil that can be compactly rolled into a relatively small volume. The formed foil is rolled and then xe2x80x9cagedxe2x80x9d in the presence of an electrolyte to grow oxide on any exposed aluminum. Aging is the process of slowly increasing the voltage on the capacitor after impregnation with electrolyte over the course of many hours by charging the capacitor using a small current source. After reaching the maximum rated voltage, the voltage is decreased and the temperature increased. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Further details of the construction of such traditional high voltage capacitors used in ICDs are described by P. J. Troup, xe2x80x9cImplantable Cardioverters and Defibrillators,xe2x80x9d at pp.704-713 (Current Problems in Cardiology, Vol. XIV, No. 12, December 1989, Year Book Medical Publishers, Chicago), which pages are incorporated herein by reference.
Alternative flat constructions for electrolytic capacitors are also known, composing a planar, layered, stack structure of electrode materials with separators interposed therebetween. An ICD with flat geometry electrolytic capacitors is described in U.S. Pat. No. 5,131,388 to Pless et al., which is incorporated herein by reference in its entirety.
The maximum rated working voltage of readily available electrolytic capacitors is in the range of 600 V. Consequently, conventional ICDs use two 350 to 450 V capacitors in series to achieve the desired high voltage for shock delivery, in the range of 700 to 900 V. In this arrangement, approximately 3 joules of energy can be stored per cubic centimeter of capacitor volume, making the capacitor the single largest limitation to further miniaturization of implantable defibrillators. From the standpoint of size, it would be desirable to provide a capacitor arrangement for an ICD in a single package rather than two capacitors in series. Thus, what is needed is a capacitor arrangement for an ICD in a single package capable of operating at a voltage of 700 to 900 volts.
The present invention is directed to an electrolytic capacitor having a novel floating anode between the cathode and the powered anode of the capacitor, resulting in a single capacitor having a working voltage double that of the formation voltage of the powered anode. The floating anode acts as cathode to the powered anode and as an anode to the cathode, such that the capacitor according to the present invention supports half the working voltage between the cathode and the floating anode and half the working voltage between the floating anode and the powered anode.
The arrangement of the cathode and anode plates according to the present invention results in a single capacitor with half the capacitance and twice the voltage of a single anode device. The capacitance lost is more than made up for by using a lower formation voltage anode foil. Additionally, the advantage of a single package and the improvement achieved using foil formed to half the intended voltage greatly outweighs the small energy density penalty for the extra thickness created by the additional anode and separator paper. For example, a foil suitable for 400 volt operation may have a maximum capacitance of 0.8 xcexcF/cm2 and is readily obtainable. The best foil suitable for 800 volt operation obtainable on the market, however, will have a capacitance no more than 0.2 xcexcF/cm2. By using two 0.8 xcexcF/cm2 foils, one as the powered anode and one as the floating anode according to the present invention, the resulting capacitance for the combination is 0.4 xcexcF/cm2, twice the 0.2 xcexcF/cm2 capacitance obtained for the foil suitable for 800 V operation. Additionally, this design is advantageous in that it will assemble in one package, giving some volume savings over two 400 V capacitors in series.